(1) Field of the Invention
This invention relates to transmissions, and more particularly to geared turbofan engines.
(2) Description of the Related Art
In high bypass turbofan engines it is advantageous that the turbine drive the fan through a reduction gearing system. This permits the turbine to operate at the relatively high speeds at which it is efficient while the much larger diameter fan operates at the relatively lower speeds at which it is efficient. Possible gearing systems include epicyclic systems. In an exemplary planetary implementation, the turbine shaft directly drives a sun gear. A number of planet gears are enmeshed between the sun gear and a ring gear that is non-rotating relative to an engine nacelle or other environmental structure in which the engine is mounted. The fan is directly driven by a carrier cage holding the planet gears. Bearings are typically provided: (a) for supporting the turbine shaft and sun gear relative to the environment; and (b) for rotatably supporting the planetary gears relative to the carrier. Whereas the former are often rolling-element bearings, the latter are advantageously journal bearings. Journal bearings may offer an advantageous balance of compactness and load-carrying ability.
In a common configuration, first and second end portions (front and rear for ease of reference) of each journal are mounted in front and rear portions of the carrier. In a basic configuration, the front and rear portions are separate plates coupled to each other by the journals and the front plate is, in turn, coupled to the fan. Under load, there may be a torsional twist of the front plate relative to the rear plate causing a journal axes to twist out of parallel with the engine axis. This misalignment can damage the gears as is noted in U.S. Pat. No. 5,466,198, the disclosure of which is incorporated by reference herein in its entirety as if set forth at length. The '198 patent teaches use of a torque frame system to prevent carrier twist. Alternatively, U.S. Pat. No. 5,102,379 teaches the presence of undercuts or annular compartments in front and rear faces of the journal to permit flexing of distal portions of the journal bearing surface relative to a journal centerline to at least partially accommodate radial loading on journals.